1. Field of the Invention
This invention relates to an improved process for the manufacture of alkylethoxy ethane sulfonates, an important class of surfactants. Specifically, a method of improving the selectivity of the sulfate-to-sulfonate reaction is disclosed.
2. Description of the Related Art
Sulfonate surfactants are well known for their stability at elevated temperatures. Unlike sulfate surfactants, these sulfonate compounds can withstand the rigors of a high-temperature, low-pH environment for extended lengths of times. Two common forms of sulfonates are alkyl aryl sulfonates (1) and alkenyl sulfonates (2): ##STR1##
An emerging technology that may require significant quantities of sulfonates is Enhanced Oil Recovery (EOR). It has been demonstrated that by adding surfactant to the water flood process of oil recovery, one can increase significantly the amount of oil recovered from a particular reservoir. For the surfactant to be effective at producing more oil, it must stay in solution within the brine phase used for the water flood. Many of the brines reinjected into the reservoir contain relatively high concentrations of divalent cations such as calcium or magnesium. Unfortunately, both alkyl aryl sulfonates and alkenyl sulfonates are sensitive to divalent cations and tend to precipitate from solution in their presence.
It is well known that this sensitivity to divalent cations can be overcome by incorporating a number of ethyl ether linkages into the molecule. The most common type of this surfactant is alkylethoxy ethane sulfates. As sulfates, the compounds do not have sufficient thermal stability to be used in all reservoirs. A less common class of surfactant, alkyl ethoxy ethane sulfonates, has both the divalent cation tolerance and the high temperature, low pH stability necessary to make it viable for nearly all reservoir conditions. One major drawback of their use is that alkylethoxy ethane sulfonates are relatively expensive surfactants to manufacture.
One common method of producing these compounds is to produce the corresponding sulfate from alcohols or phenols, then convert it to the sulfonate by reaction with sodium sulfite. This is thus a three step process:
(1) R--OH+nC.sub.2 H.sub.4 O.fwdarw.R(EO).sub.n OH PA1 (2) HSO.sub.3 Cl+R(EO).sub.n OH.fwdarw.R(EO).sub.n OSO.sub.3 H PA1 (3) R(EO).sub. OSO.sub.3 H+Na.sub.2 O.sub.3 S.fwdarw.R(EO).sub.n OSO.sub.2 Na
where
R=an alkyl or phenyl group PA2 n=an integer, generally between 0 and about 10 PA2 EO=ethylene oxide (C.sub.2 H.sub.4 O)
A major factor in the cost of producing sulfonates by this route is the low selectivity of the conversion of the sulfate to the sulfonate (step 3). Any improvement in the selectivity of this step would result in a lower cost sulfonate. In the process, the sulfating agent alternatively may be SO.sub.5.